Abstract Knowing the characteristics, connectivity patterns, and functional impacts of different cell types in vivo is necessary for understanding cortical circuits, which have been implicated in several neurological diseases. Inhibitory GABAergic neurons play an important role in increasing the functional capabilities and diversity of cortical circuits. Inhibitory cortical neurons can be divided into non-overlapping subtypes based upon their expression of parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP). SOM and PV- expressing neurons have been previously implicated in surround suppression through lateral interactions and cortical gain control, respectively. More recently, it has been shown that neurons expressing VIP are recruited during specific behaviors in mice, such as running. These cells preferentially inhibit the abovementioned SOM- expressing neurons in layer 2/3 of the primary visual cortex (V1), which leads to disinhibition of excitatory neurons and a significant increase in cortical activity. While these previous studies non-selectively activated all VIP+ neurons together, in vitro work from our lab has demonstrated that VIP neurons can actually be separated into two distinct subsets based on their expression of calretinin (CR): VIP+/CR+ and VIP+/CR- and that each of these subsets preferentially targets different neurons. VIP+/CR+ neurons, which make up approximately 40% of the VIP-expressing population, target SOM+ cells in superficial cortical layers. This proposal aims to study the effects of selective optogenetic activation of these two subsets of neurons in vivo in order to better understand the circuit mechanisms at play. This question will be addressed by utilizing Flp and Cre double transgenic mouse lines (CR-Cre/VIP-Flp) and an intersectional adeno-associated virus (AAV) that expresses Channelrhodopsin-2 (ChR- 2) under the control of Flp and Cre recombinases. After separating the VIP neuron population into VIP+/CR+ and VIP+/CR- subpopulations, extracellular electrophysiological recordings will be performed in V1 of awake, behaving mice using silicon high-density electrode arrays and record the single unit activity and local field potential in response to periodic drifting gratings of varying orientations, sizes, and spatial frequency. These experiments, which will be performed during simultaneous optogenetic photoactivation of each subset of inhibitory neurons, will demonstrate the functional effect of each cell type on visual responses. Subsequent analysis and experiments will separate the effects by cortical layer to determine whether CR+ and CR- neurons have different neuronal targets in each cortical layer, as suggested by previous literature. Finally, the activity of each of these subsets of neurons will be visualized during running and stationary behavior to view the effect of cortical state on each subpopulation of neurons and whether the increase in neuronal activity previously attributed to all VIP+ neurons is actually a result of a smaller subgroup of neurons. All in all, the proposed work will be valuable in understanding the functional connectivity and roles of specific inhibitory cortical neuron subtypes and how activation or dysfunction of these neurons affects visual processing.